1. Field of the Invention
This invention relates to foil bearings particularly used for a turbocharger, a high speed centrifugal separator and the like, which are rotated at a high speed.
As one of the air bearings, a foil bearing which employes a plurality of thin metal plates has been known. The foil bearing comprises a damp foil disposed at the inner periphery of a bearing housing for supporting a rotary shaft therein and a top foil disposed at the inner periphery of this damp foil. The rotary shaft is supported by the inner peripheral surface of the top foil in a slidable manner.
The foil type of air bearing is simple in its structure and is produced economically. Therefore, it has been applied to a small-sized gas turbine, a turbo-machine or the like in its practical use.
2. Description of the Prior Art
In the conventional foil bearing, the damp foil is composed of a thin metal plate of a corrugated shape. The corrugated thin metal plate is formed into a cylindrical shape. Whereas, the top foil is also formed into a cylindrical shape by bending a thin metal plate of a rectangular shape. The cylindrical top foil thus formed has an inner diameter slightly larger than an outer diameter of the rotary shaft.
According to this conventional foil bearing, when the rotary shaft is rotated at high speed within an axial hole defined by the inner periphery of the top foil in the foil bearing, the center axis of the rotary shaft is slightly moved from the axis of the top foil in the narrow space formed between the inner peripheral surface of the top foil and the outer peripheral surface of the rotary shaft. As a result, the rotary shaft approaches one side portion of the inner peripheral surface of the top foil thereby to form a space of a wedge shape between the inner peripheral surface of the top foil and the outer peripheral surface of the rotary shaft. Air within the axial hole in the bearing housing is sucked into the wedge-shaped space thus formed due to the rotation of the rotary shaft. In this case, the nearer the top end of the aforesaid wedge-shaped space, the higher the pressure of air sucked thereinto. The rotary shaft is forced by this higher pressure of air so as to return to the center axis of the top foil. Therefore, when the rotary shaft is rotated at a high speed, such a force acts on the rotary shaft and the rotary shaft can return adjacent to the center axis of the top foil, at all times. As a result, the outer peripheral surface of the rotary shaft is not in contact with the inner peripheral surface of the top foil, because there is formed a predetermined narrow space therebetween. Therefore, the rotary shaft is substantially supported by the air within the aforesaid narrow space thereby to have a low coefficient of friction.
However, a turbocharger for an automobile is required to be rotated at an extremely high speed. (In this connection, a gas turbine is run at 60,000 to 70,000 r.p.m., but a turbocharger is run at more than 100,000 r.p.m.) In the conventional foil type of foil bearing, such cannot provide a sufficient damping effect of the rotary shaft when rotated at such an extremely high speed. Therefore, the foil bearing is directly in contact with the rotary shaft because of whirling of the shaft. As a result, seizure is caused between the foil bearing and the shaft. Thus, the conventional foil bearing has the aforesaid disadvantages.